Control panels often have background illumination in order to expose graphics or optical indicia that identify the functions related to particular controls on the control panel. The traditional choice for illuminating control panel graphics located above, on or below a button or switch actuator, particularly in motor vehicle audio entertainment systems, has been an arrangement of incandescent lamps and light pipes, for example, as shown in U.S. Pat. No. 4,449,024. A light pipe may be formed as a transparent or translucent panel but must be specially constructed with reflecting baffle surfaces and the like to direct light rays in particular directions at various distances and locations from an incandescent light source. Thus the substantial engineering, design and implementation of the light pipe requires a long lead time, particularly with a complex light path arrangement for audio system control bezels. Accordingly, such structures were correspondingly expensive. However, they do not offer a balance of light color and intensity throughout the light pipe and they provide little or no differentiation of button actuator function graphics. For example, the illumination is unable to distinguish the operable controls of an audio system bezel or control panel during operation of a radio receiver from the operable controls and other functions during operation of a cassette tape player or a compact disc player once a different operating mode has been selected on the control panel. Furthermore, the light reflective surfaces arranged to reflect light toward a push button face after traveling transversely behind the buttons through the light pipe introduce illumination losses that interfere with identification of indicia, distinction between the indicia, and a user's selection of desired control.
Other known lighting structures include background illumination for translucent or clear button grids where a light source provides background lighting for indicia in the button. For example, U.S. Pat. Nos. 5,138,119 and 5,149,923 to Demeo disclose tactile dome switches with an illumination diode positioned beneath each dome. In other instances, the illumination is provided by an electroluminescent panel providing back lighting for the indicia. For example, U.S. Pat. Nos. 4,060,703, 4,320,268 and 4,532,395 disclose keyboard panels in which an electroluminescent panel provides background illumination for the push button indicia. Nevertheless, a luminescent panel does not distinguish one set of push buttons from another, even where the push button may be provided with multiple functions in different operating modes of the apparatus.
Moreover, in view of the complicated structures and the numerous layers of transparent or translucent material through which the light must be transmitted and controlled, previous switch constructions have suffered from mechanical disadvantages. For example, the tactile response or feel of the switches may not be as positive as is desirable for human interaction with the switches. Moreover, tactile response may be unrelated to switch closing contact, for example, when contact is made between the button contacts and a printed wiring board terminal or internal switch terminal. As a result, even though electroluminescent lamp panels have been used for back lighting keyboards, previously known switches using such panels have been difficult to build with the positive feel that is desired to complement complete switch actuation. For example, such feel is readily available in an opaque rocker switch with a domed resilient support or a push button with a dome support that provides a desirable tactile response. Accordingly, many engineering tradeoffs required a compromise between illumination clarity and switch reliability or operability.